Charged particle detectors are used in many applications requiring, for example, ion or electron detection. One such application is mass spectrometry. Mass spectrometers are widely used to separate and analyse charged particles on the basis of their mass to charge ratio (m/z) and many different types of mass spectrometer are known. Whilst the present invention has been designed with Time-of-flight (TOF) mass spectrometry in mind, the invention is applicable to other types of mass spectrometry as well as applications other than mass spectrometry which require the detection of charged particles, e.g. electron microscopy.
Time-of-flight (TOF) mass spectrometers determine the mass to charge ratio (m/z) of charged particles on the basis of their flight time along a fixed path. The charged particles, usually ions, are emitted from a pulsed source in the form of a short packet or bunch of ions, and are directed along a prescribed flight path through an evacuated region to an ion detector. The ions leaving the source with a constant kinetic energy reach the detector after a time which depends upon their mass, more massive ions being slower. TOF mass spectrometers require ion detectors with, amongst other properties, fast response times and high dynamic range, i.e. the ability to detect both small and large ion currents including quickly switching between the two, preferably without problems such as detector output saturation. Such detectors should also not be unduly complicated in order to reduce cost and problems with operation.
Conventional ion detectors for TOF mass spectrometry comprise secondary electron multipliers, such as discrete or continuous dynode electron multipliers (e.g. micro-channel plates (MCP)). In many TOF applications, e.g. requiring the detection of high molecular weight compounds, high kinetic energies for the detected ions are needed in order for the ions to be efficiently converted to secondary ions and electrons, which can be further multiplied and detected. There are two main ways of producing high kinetic energy ions for detection in TOF mass spectrometry: (i) accelerating the ions to a high kinetic energy at the detector (e.g. by applying a high voltage such as 10-20 keV to the detector) and (ii) post-accelerating the ions prior to detection. Complications may arise from the necessary complexity of electronics which this entails, e.g. where the detector is required to float at many keV potential, and the high voltage has an effect on the detector output. One solution which has been proposed is to decouple the detector output from the detector and thereby from the high potentials by converting the electrons produced by the electron multiplier detector to photons by using a scintillator and detecting the photons using a photomultiplier. Examples of such detectors are described in U.S. Pat. No. 3,898,456, EP 278,034 A, U.S. Pat. No. 5,990,483 and U.S. Pat. No. 6,828,729. However, such detectors suffer from a relatively poor dynamic range.
An optimised ion-to-photon detector has been disclosed by F. Dubois et al (Optimization of an Ion-to-Photon Detector for Large Molecules in Mass Spectrometry; Rapid Comm. Mass Spectrom. 13. 1958-1967 (1999)) in which a post-acceleration of secondary electrons is used immediately prior to the scintillator. The detector uses a faraday collector prior to secondary electron production to intercept a portion of the incoming ion beam in order to calibrate the response of the phosphor rather than improve the dynamic range. Accordingly, this arrangement still has a dynamic range which could be improved and the approach of intercepting a portion of the beam prior to scintillation tends to reduce the ultimate sensitivity.
Proposed solutions to the problem of detector dynamic range in TOF mass spectrometry have included the use of two collection electrodes of different surface areas for collecting the secondary electrons emitted from an electron multiplier (U.S. Pat. Nos. 4,691,160, 6,229,142, 6,756,587 and 6,646,252) and the use of electrical potentials or magnetic fields in the vicinity of anodes to alter so-called anode fractions (U.S. Pat. No. 6,646,252 and US 2004/0227070 A). Another solution has been to use two or more separate and completely independent detection systems for detection of secondary electrons produced from incident particles (U.S. Pat. No. 7,265,346). A further solution has been the use of an intermediate detector located in the TOF separation region which provides feedback to control gain of the final electron detector (U.S. Pat. No. 6,674,068). The problem with the latter detection is that it requires fast change of gain on the detector and it is also difficult to keep track of the gain in order to maintain linearity. A still further detection arrangement proposed in US2004/0149900A utilises a beam splitter to divide a beam of ions into two unequal portions which are detected by separate detectors. A still further arrangement using a beam splitter and a scintillator is disclosed in WO 2009/027252 A2. Methods of combining two detector outputs are disclosed in WO 2009/027252 A2, US 2002/0175292 and U.S. Pat. No. 6,646,252. In all, these detection solutions can be complicated and costly to implement and/or their sensitivity and/or their dynamic range can be lower than desired.
An arrangement for position detection in TOF mass spectrometry is described in U.S. Pat. No. 5,969,361, which comprises a plurality of electrodes embedded in a phosphorescent layer, the electrodes being used to determine where on the detector the original ions impacted.
Accordingly, there remains a need to improve the detection of charged particles. In view of the above background, the present invention has been made.